Royal Society. 149 



and the quotient, being of the second degree, shows the value of r 

 in terms of a, b, &c, which separates the real from the imaginary- 

 roots. This value is the maximum or minimum required, and the 

 equations are then numerous enough to determine r in terms of the 

 coefficients of \px. The author applies this method to cases as high 

 as the sixth degree, with quantities of two terms, and then takes 

 various problems in which more variables than one are found. 



If it were given to one of our mathematicians to make mx b — x 6 a 

 maximum without any use of hypothetical increments added to hy- 

 pothetical values, that is, without any use of the principle of the 

 differential calculus, he would soon do justice to the ingenuity of the 

 Delhi teacher ; and this though he might smile at two pages of 

 algebra substituted for two lines of the higher analysis. But the 

 student of history has seen the use of compelling investigative power 

 to work under restrictions. And the denial of tools of one kind has 

 always been the stimulus to the improvement of others. 



XXIV. Proceedings of Learned Societies. 



K.OYAL SOCIETY. 



[Continued from p. 71 •] 



Jan. 15, rf^HARLES WHEATSTONE,Esq., F.R.S., delivered the 

 1852. >->' Bakerian Lecture, "Contributions to the Physiology 

 of Vision." — Part II. On some remarkable, and hitherto unobserved, 

 phenomena of Binocular Vision. 



The first part of these researches was communicated to the Royal 

 Society in 1838, and published in the Philosophical Transactions for 

 that year. 



The second part, now presented, commences with an account of 

 some remarkable illusions which occur when the usual relations 

 which subsist between the magnitude of the pictures, on the retinae 

 and the degree of inclination of the optic axes are disturbed. Under 

 the ordinary circumstances of vision, when an object changes its 

 distance from the observer, the magnitude of the pictures on the re- 

 tinae increases at the same time that the inclination of the optic axes 

 becomes greater, and vice versa, and the perceived magnitude of the 

 object remains the same. The author wished to ascertain what 

 would take place by causing the optic axes to assume every degree 

 of convergence while the magnitude of the pictures on the retinae 

 remains the same ; and, on the other hand, the phenomena which 

 would be exhibited by maintaining the inclination of the optic axes 

 constant while the magnitude of the pictures on the retinae continu- 

 ally changes. To effect these purposes, he constructed a modification 

 of his reflecting stereoscope ; in this instrument two similar pictures 

 are placed, on moveable arms, each opposite its respective mirror; 

 these arms move round a common centre in such manner that, how- 

 ever they are placed, the reflected images of each picture in the mir- 

 rors remains constantly at the eame distance from the eye by which 



